The number of direct broadcast services from satellites continues to increase, particularly in the United States. There is, therefore, an expanding market for low cost ground terminals operating at a number of frequencies in the microwave region. Currently, these involve the use of reflector antennas with diameters typically in the range from 0.5 to 2 meters. Because of their simplicity, these antennas currently represent the lowest cost option. In today's developing market, several design alternatives are currently being explored including the use of antennas made from flat plate arrays.
The market for earth stations with antennas under 2 meters in diameter continues to grow. Applications for such antennas include business communications for data collection or dissemination, remote control and inventory management with projected network sizes ranging from a few hundred terminals to tens of thousands of stations. Typically, the system is based around a 12/14 GHz satellite service with a master station (employing a 5 to 10 meters antenna and high power amplifiers) making up for the relatively low gain of the "micro" earth station.
Many of the systems currently under development employ reflector antennas. These are simple to produce using a variety of techniques and there is considerable competition resulting from the large number of manufacturers entering the market, particularly in the United States where the development is most rapid. It has generally been assumed that flat plate array technologies, despite many attractive features, are not viable when the dimensions are greater than, say, 1.0 meter because of the increased complexity and, therefore, manufacture and assembly costs, a large proportion of which is due to the need to connect the cavity exciter probe with the feeder board. However, it is now evident that the cost differential can be made sufficiently small (by using, for example, inexpensive materials and injection molding techniques) and could be more than offset by the size reduction and the aesthetic appeal of a flat plate array.
Early flat plate array designs were based on microstrip configurations etched on high performance plastic substrates. These employ patch radiators, fed from a co-planar microstrip power dividing network or comb-lines which consist of a series of open-circuit stubs directly connected to a single microstripline. Although suitable for low or medium gain applications where a low profile is essential, they have a number of disadvantages. These include:
(1) The high cost of low loss substrates (e.g., glass fiber reinforced Teflon or PTFE).
(2) The narrow bandwidth of the radiators (which are generally operated about resonance).
(3) The increase in losses with frequency which become unacceptable in larger arrays.
(4) The poor sidelobe performance resulting from spurious radiation associated with the microstrip feed network.
Although improvements have been made using, for example, multilayer configurations incorporating shielded feed networks and sandwich support structures for the radiating elements, experience suggests that the technology is most appropriate at frequencies up to, say, 5 GHz and difficulties in scaling limits its applicability to the 12/14 GHz bands.
A number of prior art array configurations have been considered for direct broadcast service applications. Initially, interest centered on designs based on an open microstrip, including arrays of patches and comb-lines. These were found to have a number of drawbacks, however, including bandwidth limitations, radiation from the open feeding network and substrate losses which become more significant as the array size is increased. Alternative designs were, therefore, pursued which are based on cavity radiators fed from low loss feeder networks in, for example, suspended stripline. These offer better performance in terms of radiation pattern control and efficiency. However, in contrast with microstrip arrays, the structure is three-dimensional and, therefore, novel manufacturing approaches are required to realize it in a low cost form.
U.S. Pat. No. 4,054,874 relates to a high frequency antenna formed from elements by means of which circularly polarized signals can be transmitted or received. Each element is assembled from a pair of conducting dipoles which are joined in a cross-wise configuration by means of their central portions to constitute one single device, coupled to the ends of corresponding transmission lines. The lengths of the transmission lines differ by 1/4 of the wavelength associated with the frequency of the transmitted or received signals in order that these useful signals are in phase quadrature.
U.S. Pat. No. 4,486,758 (de Ronde) relates to an antenna element for circularly polarized high frequency signals. The element includes a pair of superposed planar dielectric layers. An outer surface of each layer is covered with an electrically conductive layer forming a ground plane and having a circular opening defining respective cavities. Orthogonally crossed dipoles are disposed between the dielectric layers and adjacent the openings for coupling radiation to the feed line through striplines also disposed between the dielectric layers.
U.S. Pat. No. 4,527,165 (de Ronde) relates to a miniature horn antenna array for circularly polarized high frequency signals. An insulating layer includes openings defined by metal plates walls forming miniature horns, each having a square cross-section. A dielectric layer adjacent the insulating layer supports a first supply network for signals whose direction of polarization is of a first type of linear polarization. A second insulating layer adjacent the dielectric layer includes openings defined by metal plated walls forming miniature waveguides each having the same square cross-section as a respective horn, at the side facing the first network, and having a rectangular cross-section at the other side. A second dielectric layer adjacent the second insulating layer supports a second supply network for signals whose direction of polarization is perpendicular to the polarization of the signals of the first network. A third insulating layer adjacent the second dielectric layer includes openings defined by metal plated walls forming miniature waveguides each having the same rectangular cross-section as a respective waveguide in the second insulating layer, at the side facing the second network, and which has a depth smaller than the thickness of the third insulating layer.
The de Ronde patent '165 provides a configuration in which the suspended stripline feed network is interleaved between the elements. The array is made up of a series of layers with the suspended stripline central conductor formed on a thin, highly flexible Kapton sheet. Coupling to the radiating elements is realized by extending the ends of the suspended stripline conductor into the cavities to form an E-field probe.
In the de Ronde '165 patent the suspended stripline transmission line consists of a thin Kapton sheet clamped between plates in which mirror imaged channels are formed. The central conductor is etched on the surface of the Kapton sheet, with a width chosen to achieve the desired characteristic impedance. A complete singly polarized array then consists of two plates (which may be either machined directly from solid aluminum or formed in plastic and then metallized) with a single sheet of Kapton between them. The top plate includes the radiating apertures and the upper section of the power dividing network. The lower portion of the power dividing network and the shortened section of the radiators are incorporated into the lower plate.
There is thus a need for an improved flat phased array antenna assembled from economic materials such as plastic by modern techniques including injection molding and yielding superior result. The present invention is directed to filling that need.